In applicant's co-pending published International Application WO 92/22938 there is disclosed a dual polarisation waveguide probe system in which a waveguide is incorporated into a low-noise block receiver in which two probes are located for receiving linearly polarised energy of both orthogonal senses. The probes are is located in the same longitudinal plane and on opposite sides of a single cylindrical bar reflector which reflects one sense of polarisation and passes the orthogonal signal with minimal insertion lose and then reflects the rotated orthogonal signal. The probes are spaced λ/4 from the reflector. A reflection rotator is also formed at one end of the waveguide using a thin plate which is oriented at 45° to the incident linear polarisation with a short circuit spaced approximately a quarter of a wavelength (λ/4) behind the leading edge of the plate. This plate splits the incident energy into equal components in orthogonal planes, one component being reflected by the leading edge and the other component being reflected by the waveguide short circuit. The resultant 180° phase shift between the reflected components causes a 90° rotation in the plane of linear polarisation upon recombination so that the waveguide output signals are located in the same longitudinal plane. Furthermore, in applicant's: co-pending International Patent Application PCT/GB96/00332, an improved dual polarisation waveguide probe system is disclosed for use with a wider frequency range transmitted by new satellite systems. In this improved probe system, a reflective twist plate was provided within the probe housing, the reflective twist plate having at least two signal reflecting-edges so that at least two separate signals reflections are created. The multiple signal reflections enable the probe system to operate over a wider frequency range with minimal deterioration and signal output.
Applicant's co-pending International Published Application PCT/GB97/02428 disclosed a further improved waveguide which is able to operate across the entire frequency band of a satellite system with substantially the same performance. In this system the waveguide included a rotator which incorporated a reflecting plate in combination with a differential phase shift portion in the form of a waveguide of slightly asymmetrical cross-section so that orthogonally polarised signals that travel through the portion have different cut-off wavelengths. This results in a signal rotator which achieves 180° phase shift between two orthogonal components across the frequency range of signals received by the waveguide. The reflecting plate and the differential phase portion have inverse phase change with frequency characteristics so that the combined phase shift characteristic of the rotator shows a flatter response across the desired frequency range.
Although these systems generally work well, they suffer from a number of disadvantages. First, a waveguide which incorporates an edge reflecting plate can incur inconsistencies over a large number of repeated castings and as the leading edge of the plate becomes thinner, it is more likely that the reflecting edge will be damaged in casting and the materials which can be suitably cast to provide such leading edges becomes limited. Furthermore, these systems are generally used with circular waveguides and it is desirable to provide an improved waveguide rotation system which can be used with other waveguide shapes such as square or rectangular which still provides suitable rotation performance. Furthermore, with such existing waveguides the overall dimensions of the waveguide housing are often determined by the waveguide. Furthermore, the use of solely circular waveguides can limit the design options for the circuit board housing and a smaller housing can be afforded by the use of a square waveguide.